This invention relates to in situ time response testing of hydraulic sensors in a nuclear reactor fluid circuit, and particularly to response time testing of pressure and differential pressure sensors in light water nuclear reactors.
The United States Nuclear Regulatory Commission (NRC) requires periodic testing of the dynamic response characteristics of hydraulic instrumentation to insure reliable and safe nuclear operation. Specific industry response time standards set forth, for example, in IEEE Standard 338-1975, entitled "Criteria for the Periodic Testing of Nuclear Generating Station Class 1E Power and Protection Systems", state in part that sensors must be periodically tested if response time thereof are shown to be critical to reactor safety or are known to comprise a significant part of the overall system response time and may be expected to suffer response time degradation. For example, the permissible overall response time of instruments to a contingency, such as the sudden loss of coolant, might be less than two seconds. Selected instruments must generally react in a small fraction of the overall response time. Failure to initiate safety procedures within the tolerable response time could cause substantial damage.
Heretofore, the nuclear industry has specified sensors especially designed and adapted for use in the nuclear environment. Close response time tolerance has been presumed from instrument design. In recent years, the nuclear industry has been permitted to specify general purpose process sensors having originally been intended for non-nuclear applications. Data respecting response time for such instruments are often unavailable or are not based on uniform criteria. In addition, available response time data are typically estimated based on laboratory rather than field conditions. Thus, there has been no guarantee that the data provided if any, are reliable.
With the recent promulgation of the NRC periodic testing requirement, it has been necessary to devise procedures and provide instrumentation to monitor and to verify sensor response time. Techniques requiring removal of the sensor from the reactor, and therefore decontamination prior to examination and testing, are considered unreliable and generally impractical, since removal and reinstallation of the instrument could result in damage to the instrument and disturbance of the system and expose workmen to hazardous doses of radiation. Nevertheless, no suitable instrument has heretofore been made known for field testing the time response of in situ reactor pressure sensors.
Reliable data collection also requires an accurate and repeatable testing technique. A number of testing techniques are suggested by transient analysis. These include the impulse response test, the step response test, the sinusoidal perturbation test, and the ramp response test. The various testing techniques enjoy particular well-established theorectical advantages and particular significant practical disadvantages. In the first three techniques, a suitable hydraulic test signal is extremely difficult to generate with any degree of reliability and repeatability for the tests in the time period of interest. (Typically, sensor time responses of interest are less than 100 milliseconds, so system time resolution must be considerably shorter). Moreover, it is quite often difficult to relate the data to actual accident conditions.
The ramp function, however, is found to be a particularly attractive test function because it approximates and simulates many of the accident orifices which might occur, for example, by a sudden loss of pressure. Furthermore, time delay of a ramp function is readily measureable in comparison with a known reference.
To generate a reliable ramp function, a hydraulic signal generator is required which is capable of producing a linear increase and decrease in hydraulic pressure, without transient disturbance, over a wide pressure amplitude range and over a broad dynamic rate range. In addition, the hydraulic generator must be capable of producing the desired signals in a field environment. A particular requirement is a capability to produce linear, transient free hydraulic ramp signals which are independent of the volumetric displacement of the sensor.
Single pressurized hydraulic (i.e., liquid-filled) or pneumatic-hydraulic (i.e., gas-liquid filled) accumulators with a solenoid gate valve throttle valve interposed between the accumulator and sensors have been suggested as a means for generating the ramp hydraulic signal. Such apparatus are found to be inadequate in the application of present interest for a variety of reasons. First, and particularly in the increasing pressure mode, shock waves transmitted through the hydraulic fluid create pressure oscillations rather than a smooth ramp at the hydraulic output. Such oscillatory transients are relatively so severe as to hamper accurate and repeatable time delay measurement.
Second, in the decreasing pressure mode, pressure is changed with such rapidity that controllability can not be attained. Repeatable and accurate time delay measurement is not possible without relatively elaborate computation and is considered impractical for the purposes of this procedure.
Third, it has been likewise found to be impossible to attain broad dynamic rate ranges with such single hydraulic accumulators because of their extreme sensitivity to the volumetric displacement of the process sensor over the single pressure range.
It is therefore an object of the present invention to provide a relatively simple apparatus and a readily understood method for measuring the time response of in situ hydraulic sensors in a reactor to verify sensor time response operability as required by nuclear safety standards.
It is a further object to provide a portable instrument which can be employed in field testing of hydraulic sensors in a nuclear reactor.
A particular object of the invention is to provide an accurate indication of time delay between a change of the hydraulic pressure input to a sensor and the corresponding output signal indication.
A specific object is to provide a hydraulic signal generator capable of producing substantially linear and transient-free increasing and decreasing hydraulic pressure ramp signals independent of the volumetric displacement of the process sensor.
A further specific object is to provide a hydraulic ramp signal generator capable of producing repeatable signals over a broad amplitude and rate of change dynamic range, and particularly a generator which is capable of repeatably producing a broad range of pressure change rates at both low and high pressures.